1. Field of the Invention
The present invention is related to binary phase shift keyed (BPSK) receivers of the type employed in very high speed aircraft and satellite vehicles. More particularly, the present invention is related to low to medium data rate receivers that incur high acquisition time due to doppler phase shift.
2. Description of the Prior Art
Prior art BPSK receivers employ phase locked loops (PLL) to control the voltage controlled oscillator (VCO) to obtain the center frequency of the carrier signal. Such prior art receivers, when not subject to doppler effect, employed PLLs which had a natural pull-in range which would permit acquisition of the carrier signal. However, low to medium data rate receivers have very narrow loop band widths or pull-in ranges that do not encompass the doppler shifting of the carrier signal for receivers that are installed in high speed aircraft and moving satellites.
Prior art low to medium data rate receivers have employed sweep circuits which change the frequency of the VCO and force the frequency of the PLL through the center frequency of the doppler affected carrier frequency. Such sweep circuits and phase locked loops suffer from two major problems. First, they were limited to sweeping the frequency of the VCO at the rate of frequency change which permits the phase locked loop to lock onto the phase of the doppler shifted frequency. Such prior art phase locked loops required narrow band width carrier recovery loops to reduce phase jitter. Such phase locked loops had band widths limited to approximately 1% of the data rate. Such small band widths required slow sweeps which caused extremely high acquisition times especially when there was a large frequency uncertainty. When a large frequency of uncertainty was encountered, a wide range frequency sweep circuit was required. Prior art frequency sweep circuits often employed base band square law detectors which generate side bands that promote phase locking on side band signals.
The second problem is inherent in phase locked loops because the phase locked loop cannot distinguish the side band signals (or false signals). When sweeping the voltage controlled oscillator in one direction, the phase lock detector is designed to indicate that a phase locked condition has occurred and attempts to lock onto the incoming signal. If the sweep circuit sweeps too fast, it is possible to overshoot the side band signal or the true center frequency signal and the acquisition time is prolonged during seeking or subsequent reverse sweeping procedures. This problem is exacerbated because there is an infinite number of side band signals and only one true center frequency signal.
The above mentioned problems have yet to be solved for receivers subject to doppler phase shift. Present high speed aircraft still communicate with each other employing FM or amplitude modulated voice channels which require pilot attention. Present high speed aircraft do not have computer systems which assimilate important information and communicate it to other aircraft at low to medium digital data rate frequencies that could be encrypted by known techniques.
It would be extremely desirable to provide a low to medium data rate receiver which has faster acquisition time than receivers known heretofore. More importantly, it would be desirable to eliminate false lock-on of side band or harmonic signals and to eliminate overshoot seek sweeping during acquisition. Such a receiver would permit rapidly moving aircraft to automatically transmit and receive valuable data.